The visual system of the giant barnacle has proved useful for studying photoreceptor synapses as well as signal detection, amplification, and adaptation, in a simple visual pathway. A total of ten huge photoreceptors, divided amongst three eyes, project to a ganglion where visual information is processed by a small number of neurons. The similarity of the physiology of the receptors and postsynaptic cells to that of cells of the vertebrate retina makes this system attractive for studying membrane and synaptic properties and circuitry underlying visual processing. We propose experiments to understand the mechanisms of synaptic transmission from these photoreceptors, where it is possible to impale the presynaptic terminal regions, control its voltage, and simultaneously impale the postsynaptic cell. We particularly hope to understand the adaptation shown to occur at this synapse. Using techniques of voltage clamp and optical recording we will ask: 1. What are the properties of the Ca channels in the presynaptic membrane? How does the presynaptic intracellular Ca concentration change with the transient and prolonged stimuli? What is the voltage profile in the terminal arbor during prolonged depolarizations? 2. What is the transmitter released by the photoreceptors? What is the size of the quantal unit released? We also propose to record from and voltage clamp second- and third-order cells to explore certain aspects of processing of visual signals. 3. How are off-responses generated in the second-order cell, particularly over a wide range of background light intensities? 4. How does signal compare to noise at various backgrounds in the receptors and second-order cells, and how does convergence improve signal detection? 5. What specific information is encoded in each of the known third-order cells and what are the mechanisms of their off-responses?